1. Field of Invention
This invention relates generally to chemical vapor deposition and, more particularly, to a particular kind of chemical vapor deposition called flame hydrolysis.
2. Description of Prior Art
In flame hydrolysis-type chemical vapor deposition, vapors of a glass-forming oxide material are directed to flow through a high-temperature, hydrogen-containing flame, whereupon the vapor reacts with water produced in the flame, to form oxide particles called soot. A portion of this soot is collected, to form a useful body of porous glass-forming material. The process is commercially used to produce optical fiber preforms and other high-purity, high-silica glass products. Often, the soot deposition occurs on a rotating substrate rod or tube, to form a cylindrical object. This process is called the outside vapor deposition or, in abbreviated form, the OVD process.
The flame hydrolysis process is described in U.S. Pat. No. 2,272,342, which issued in the 1940s, and the OVD process has been described in detail in chapter 2 of a book entitled “Optical Communications, Volume 1, Fiber Fabrication,” edited by Tingye Li (1985). While depositing clad glass on a core rod for manufacturing clad glass for optical fiber preforms by the OVD process, the target core rod typically has a diameter between 20 and 50 mm. Uniform-diameter silica soot is deposited onto the rod, layer by layer, by traversing or oscillating an array of burners along the rod's entire useful length, until a diameter in the range of 150 to 400 mm has been achieved. Past equipment for performing this process has always used the same burner array throughout the deposition process, for the entire range of diameters. The burners of each such burner array have been optimized for maximum deposition rate and efficiency over the entire diameter range.
A specific burner configuration can achieve a maximum deposition rate only over a limited diameter range. To increase deposition rate beyond this maximum rate, multiple burners in an axially separated burner array have been used. Many configurations for such burner arrays have been used. In a burner traverse deposition apparatus, an array of burners traverse along the entire length of a rotating, but axially stationary target rod. In a preform-traverse deposition apparatus, on the other hand, the burners are held stationary and the rotating target rod traverses along the entire length of the burner array. Further, in an oscillating burner deposition chamber, an array of burners traverses a distance greater than the separation of adjacent burners of the array, but shorter than the length of the rotating target rod.
FIG. 1 shows one example of a prior art preform traverse apparatus 10, incorporating two burners 12 arranged in a vertically oriented array. A rotatable chuck 14 is mounted on a traverse mechanism, so that the chuck and a support rod 16 are moved up and down while rotating. The support rod projects downward into a deposition chamber 18, and it supports a hanging mechanism 20 from which is hung a target rod 22. A handle rod 24 is located at the target rod's lower end. Two flame hydrolysis burners 12, forming a two-burner array with the burners vertically separated, are mounted on an array mount 26, which in this case is shown as stationary in the vertical direction. The deposition process begins by moving the burners 12 horizontally off-axis and supplying chemicals to the burners for ignition. After the flame has stabilized, the burners are moved horizontally into a deposition position, in which the flames point directly at the target rod's axis of rotation 28. FIG. 1 also shows a coaxial alignment between the burners, an air intake, and an exhaust airflow. As the target rod traverses upward and downward in front of the burner array, the glass-forming particles, called soot, formed in the burner flames, deposit onto the target rod, forming layers of porous soot body or preform 30. After the weight of the porous glass has reached its target value, the cylindrical soot preform is raised above the deposition zone, the burners are turned off, and the preform is removed from the hanging mechanism 20 in the deposition chamber 18.
One limitation of the prior art machines described above is that burners of a single design are used throughout the deposition process. However, a flame hydrolysis burner design that is optimized for deposition onto small-diameter target rods is not necessarily efficient in depositing onto large-diameter target rods; conversely, a burner design optimized for deposition onto large-diameter target rods is not necessarily efficient in depositing onto small-diameter target rods. The prior art approach of using one burner design for the entire diameter range of deposition is becoming competitively limiting, as the need for ever-larger diameter preforms continues to increase.
Another limitation of the prior art machines described above is that burner spacing is fixed for the entire diameter range of deposition. However, as the target rod diameter increases during deposition, the soot stream and the flame spread axially over a longer length as they flow across the target rod. This requires a larger axial separation of burners to prevent adjacent burners from interacting with each other and negatively affecting the deposition rate and efficiency.
A vertical preform orientation is more convenient for deposition of large soot bodies, because it eliminates bending of the deposited object under the force of gravity, as can occur in a horizontal preform orientation. Nevertheless, one limitation of a deposition chamber having a vertical preform orientation and a coaxial burner and exhaust configuration is that a dead zone of zero velocity can be formed on the target rod's surface, 180° from the burner, where opposing soot streams flowing across the target rod collide with each other. Soot trapped in this dead zone because of buoyancy can float upward and deposit outside the hot zone and onto different parts of the deposition chamber's inner surface. Deposition outside the hot zone should be avoided, because the deposited material has a relatively low density and can cause the preform to crack. In addition, down time of the machine between runs is increased if soot has deposited onto the deposition chamber's inner surface, as that surface needs to be cleaned before starting the next deposition cycle.
It should, therefore, be appreciated that there remains a need for an outside chemical vapor deposition apparatus that deposits porous glass-forming material onto a target rod to form a cylindrical body, with greater efficiency than could be achieved in the past. The present invention fulfills this need and provides further related advantages.